In recent years, despite high definition of an image display device, transmittance has been lowered while the total luminous flux of a light source has been increased. In order to increase the luminous flux, the number of light sources to be attached is increased or the power per one light source is increased, but in either case, power consumption of the entire light source is increased. Comparing a direct type in which the light sources are arranged on the whole backlight chassis and an edge light type in which the light sources are arranged on a side surface the backlight chassis as the arrangement of the light sources, the direct type is advantageous in terms of heat radiation, and a high definition image display device often uses a direct type backlight.
In the direct type backlight, a plurality of rectangular substrates to which a plurality of light sources are attached are arranged. As the light source, a light emitting diode (LED) or the like is used, and from the viewpoint of improving heat radiation, a metal plate such as aluminum is often used for rectangular substrate. An example of such a system is disclosed in Japanese Patent Publication No. 5351723.
However, when the metal substrate such as aluminum is used for the rectangular substrate, it is difficult to form a wiring pattern on a back surface of the substrate and it is difficult to specify a position of the LED from the back surface because the metal is exposed on the back surface of the substrate on which the LED is not mounted. Therefore, it is necessary to dispose a heat radiation member on the entire back surface. Further, when the backlight chassis is a metal such as iron, coefficients of thermal expansion of the aluminum used for the substrate and the iron used for the chassis are different. In this case, if a heat radiation tape with a double-sided adhesive property is used for the heat radiation member, the aluminum substrate may be peeled off and float from the iron chassis.